A wing constituting an airframe of an aircraft is of a hollow construction, and a wing surface panel forming a wing surface is fixed to a structural member existing in the wing by a fastener member (fastener).
The fastener member fastens the wing to the structural member by inserting a pin-shaped fastener main body into through holes, which have been formed in both the wing and the member attached to the wing, from the outside of wing, and by fixing the front end portion of the fastener main body by using a fixture from the interior side of the wing.
The aircraft must make thoroughgoing provisions against lightning strikes. In the case where the wing surface panel and the fastener member are made of different materials, at the time of lightning strikes, an arc discharge (spark) is produced in the direction extending along the interface between the wing surface panel and the fastener member by an electric potential difference between the wing surface panel and the fastener member. Since a fuel tank is housed in the internal space of the wing, it is necessary to reliably suppress the production of arc discharge at the time of lightning strikes.
To meet such necessity, a structure has conventionally been proposed in which, as shown in FIG. 9, on the inside of a wing 1, a cap 6 is installed in a state of being separate from a fastener main body 4a of a fastener member 4, which penetrates a first member 2 corresponding to the wing surface panel and a second member 3 installed within the wing, and a fixture 4b, and an air-gap 7 filled with air is formed between the cap 6 and the fastener main body 4a, the fixture 4b (for example, refer to Patent Literature 1).